This invention relates to ribbon cartridges, and more particularly, to a ribbon cartridge with a self-contained ratchet and drive gear assembly for winding used ribbon back into the cartridge.
Ribbon cartridges normally include a supply reel for holding a supply of fresh ribbon and a take-up reel to which used ribbon is transferred. A capstan located on an external apparatus, e.g., a printer, withdraws the ribbon from the supply reel, transports the ribbon at a constant rate past the print head of the printer, and retracts the ribbon back into the cartridge so that it may be wound on the take-up reel.
In an unused cartridge, the amount of ribbon wound on the take-up reel is small. However, as the ribbon is supplied from the supply reel, the amount of ribbon being wound on the take-up reel increases steadily. Eventually, all the ribbon is removed from the supply reel, after which the cartridge is discarded or, with some cartridges, the ribbon can be rewound on the supply reel for reuse.
In light of the above, means must be provided for driving the take-up reel as well as the capstan, such that the ribbon progressively and efficiently accumulates on the take-up reel, without snarling. Care must be taken, however, to ensure that the rate at which the ribbon is wound onto the take-up reel does not exceed the rate at which the ribbon is drawn into the cartridge by the capstan. If this latter rate is exceeded, an undesirable pulling force is exerted on the ribbon.
Efforts to date to design a simple, effective and economical drive assembly providing a constant rotational driving force for the capstan and a variable rotational driving force for the take-up reel have not been totally successful.
For example, most ribbon cartridges currently on the market utilize an O-ring drive assembly, wherein a flexible O-ring is stretched between projections formed on a drive gear and the take-up reel, which projections protrude through the cartridge cover. At the beginning of operation of such an O-ring drive assembly, the revolutions per minute of both the drive gear and the take-up reel are approximately the same. As the diameter of ribbon accumulating on the take-up reel increases, however, the take-up reel has to turn slower, which is accomplished through slippage at the interface of the projections and the O-ring. In theory then, near the end of operation of the cartridge, the drive gear turns many revolutions for every revolution of the take-up spool.
A major drawback of the O-ring drive assembly is that, as the O-ring slips, it generates unwanted debris. In addition, the O-ring has been known to actually break due to wearing through. Further, the physical characteristics of the O-ring are very critical in that the length, durometer, material composition, etc., must be very tightly controlled to assure that the O-ring slips just enough, but not too much. Accordingly, O-ring drive assembly design and manufacturing is necessarily expensive and time-consuming, without dependable operation being assured.
U.S. Pat. No. 3,967,790, issued to HESS, teaches another type of drive assembly for a ribbon cartridge. This patent teaches the use of a slippage gear clutch drive mechanism for providing a direct drive for the capstan and a releasable driving force for the take-up reel. When the pulling force generated by the increased accumulation of ribbon on the take-up reel exceeds the magnetic coupling force between a first gear and a second gear, the first gear slips relative to the second gear until the ribbon tension drops below a threshold value. When this occurs, the second gear is again rotated by the first gear, thereby driving the take-up reel. The magnitude of the threshold force for the slippage is primarily dependent on: the strength of the magnet; the magnetic permeability of a drive ring and an annular magnetic member used therein; the number and dimensional configuration of the teeth on the gears; and the air gap therebetween.
As can be seen, the drive assembly of the '790 patent is located outside of the cartridge. Therefore, the clutch mechanism in the drive assembly has to last for the life of a great many cartridges. Accordingly, the clutch mechanism must be built to be very durable and, therefore, manufacturing costs are high.
In light of the above-discussed prior art, a need still exists for a ribbon cartridge drive assembly which is capable of the most cost-efficient production and the highest operational reliability.